bottlefed 13 wk old loss of appetite

My 13wk old is only having 3 or 4 feeds of 4 oz a day! She is gaining weight though I dont know how. She was taking 5 lots of 6oz until 2 wks ago & was sleeping through. Now she is waking lots in the night for 1 or 2oz and then off to sleep again. Dont know what to do, health visitor says she is fine. Has anyone else experienced this? should we see the GP?